1. Field of the Invention
The present invention generally relates to the phenomenon of silver migration on circuit boards or chips, and more specifically to providing conductive rims around the periphery of silver contacts to prevent silver migration.
2. Description of the Related Art
Silver migration is a well known, if not completely understood, problem in electrical circuits that use exposed silver metalization for electrodes, leads or bump connections hereafter referred to as contacts. Silver provides the best electrical and thermal conductivity characteristics for circuit contacts, and is relatively inexpensive, but has the highest propensity to migrate. A silver contact's surface oxidizes when exposed to the atmosphere, creating a thin oxidation layer that exhibits semiconductor properties and has relatively weak bonds between the positively charged silver ions Ag.sup.+ and the neighboring silver or silver oxide atoms. Applying bias voltages or transmitting voltage signals via a pair of contacts creates potential differences between the contacts and establishes electric fields at their semiconductor-like oxidized surfaces. These electric fields can ionize the positive contact's (anode) surface and accelerate the Ag.sup.+ ions towards the negative contact (cathode) where they recombine with free electrons, causing an electrical short between the contacts. In addition, the migrating silver ions could interfere with other circuitry on the board.
FIG. 1 is a sectional view of a circuit board 10 that is provided with silver contacts 12 and 14 that oxidize when exposed to the atmosphere to form thin layers of silver oxide 16 and 18 on their respective surfaces. Applying a voltage from a voltage source 20 across the contacts produces an electric field at their surfaces. The voltage source 20 can represent the potential difference between otherwise electrically isolated contacts, resulting from ac voltage signals or bias voltages on respective contacts. The surface of the anode's oxidation layer 16 can ionize if the field is strong enough, and the positively charged silver ions will then migrate towards the cathode.
Other metalization materials such as gold and titanium do not have significant migration problems, but are electrically and thermally inferior to silver. Titanium oxide is a very stable compound with strong ionic bonds and remains slightly conductive. Gold does not oxidize when exposed to atmosphere, and hence will not ionize.
Silver migration has been studied for many years without solution. To reduce or avoid the problem Licari and Enlow, "Hybrid Microcircuit Technology Handbook", Noyes Publications, pp. 102-106, (1988) suggests reducing the electric fields between critical components by limiting the signal voltages and increasing component spacing, using silver alloys (Pt or Pd) which increase resistance substantially, or hermetically sealing the components to prevent oxidation. These constraints complicate the circuit design, diminish the contacts' electrical properties and increase manufacturing costs. As a result, silver contacts are rarely used. Instead Au, Cu, Pt or Al are generally used for the various metalization requirements in electrical circuits.